Inverse feedback amplifier system



May 26, 1942. D. s. BOND INVERSE FEEDBACK AMPLIFIER SYSTEM Filed Oct. 31. 1939 y 4, K ZSnentor Donald 5.

Patented May 26,- 1942 Donald S. Bond,

ltadio Corporation of v of Delaware Philadelphia, Pa., assignor to' America, a corporation Application October 31, 193i), Serial No. 302,195 1 Claim. (Cl- 179-171) The present invention relates to inverse feedback amplifier. systems and has 'forits primary object to provide improved coupling means for a feedback circuit of an amplifier of the character referred to whereby inverse feedback may be obtained with a minimum'number of circuit components.

It is also an object of the present invention to provide an improved and simplified inverse feedback amplifier circuit including a pair of cascadeconnected amplifier stages in which inverse feed back and anode currents are supplied to a preceding stage through a common anode circuit, thereby simplifying the circuit and reducing the number of circuit components to a minimum. I The invention will, however, be better understood from the following description when considered in conjunction with the accompanying drawing, and its scope is pointed out in the appended claim. I

Referring to the drawing,

' Figure 1 is a schematic circuit diagram of an inverse feedback amplifier system embodying the invention, and

' Figure 2 is a similar schematic circuit diagram showing a modification of the invention.

Referring to Fig. l, is an audio-frequency amplifier tube having an input circuit indicated by terminals 5 and I, the former being coupled to the input grid 3 through a coupling capacitor 3 and the latter terminal being connected to ground or chassis iii. A biasing potential is supplied to the grid 8 through a grid resistor connected to ground and through ground to the negative end of a self-bias resistor l2 in the cathode lead of the tube. Y The amplifier tube anode indicated at I3 is coupled through a suitable coupling network ll with the input grid l5 of a second stage amplifier tube It, the network shown comprising a coupling capacitor H, and anode and. grid circuit impedances I8 and I9 respectively of the usual impedance coupling network, although it should be understood be employed, as will hereinafter be shown.

The coupling impedance I8 is connected with the anode I3, while the grid impedance |9 is connected withthe grid l5 and with ground It, and

thence to the negative end of a second self-bias resistor 20 in the cathode lead of the tube IS.

The output circuit for the second stage amplifier tube It comprises the anode 2|, a lead 22', and the primary 23 of an output transformer 24, across a secondary 25 to which is connected any suitable load indicated by a resistor currentis supplied to the anode 2| through the transformer primary 23 from a supply lead 21 connected with a positive supply terminal 28. A bypass 29 to ground is provided for the supply lead 21 as a filter capacitor and as an alternating current low impedance path to'ground and to the cathodes of amplifier tubes. This path is completed to the cathodes through suitable bypass capacitors 30, as shown.

The usual inverse feedback amplifier circuit includes a connection from the output circuit of one stage to the input circuit of another stage through suitable coupling elements. In accordance with the invention, however, in the present example, the low signal potential end of the coupling or anode impedance I8 is connected to the anode side of the coupling transformer primary 23, as indicated at 32. Thus, both anodes l3 and 2| draw current through the primary with the grid that a transformer coupling may 20'. Anode plied to-the grid l5.

winding 23 from the-positive supply lead'2l, it being understood that ground or chassis represents the negative B supply connection.

With this arrangement, the output anode circuit of the tube 5 comprises the impedance element l8 and the impedance element 23 in series together with the bypass capacitor 23 to ground. The feedback circuit from the output anode 2| includes only the impedance element 23. Thus, the amount of feedback obtained is dependent upon the relative impedances of the elements and 23.

= Broadly, through an equivalent series impedance, which may be the plate impedance of the tube 5,. into avoltage divider which comprises the said plate impedance and the plate load impedance l8 of the following tube i6 connected to a tap on the divider and with the high impedance feedback circuit connected to the same or a different tap, such as the tap 32. The tube impedance of the tube 5 and the plate load im pedance III in series constitute a common voltage divider for both the signal amplified by the tube 5 andthe feedback voltage developed across .the impedance 23 in the plate circuit of the second stage l6. It is preferable that the plate impedance of the tube l6 be greater than the sum of the impedances of the element l8 and of the tube 5. Normally, impedance I9 will be chosen considerably larger than impedance Iii under which conditions the fraction of the voltage developod across the impedance 23 which is apof tube I6 is equal to the ratio of the plate impedance of tube 5 to the sum of this impedance and-the impedance of l8.

this-involves coupling the input signal Referring now to the modification shown in Fig. 2, the input or driver stage 8 is shown transformer-coupled to a pair of balanced output amplifier tubes 38 and 88. In this arrangement, the inputterminals 8 and 1 are coupled through a coupling capacitor 8 to the control grid 8 of the The outer terminals .of the secondary 39 are 41 connected with the signal input grids 43 of the tubes 35 and 36. The tubes are shown as being of the screen grid type having screen grids 44 connected to the positive anode supply lead 45.

The anodes 48 are connected in balanced relation to each other through a balanced primary winding 41 of an output transformer 48. The

. center tap 49 is connected with the lead 45 to supply anode current to the anodes 48. The secondary 50 of the transformer is connected with a suitable load device such as a loudspeaker The inverse feedback voltage divider circuit is provided in the present example by connecting the anode current supply lead 52 for the anode circuit of the tube 5 with one of the anode terminals of the primary 41 as indicated at 53 or with a suitable tap 54 on theprimary 41, as indicated by the dotted connection 55, so that the resultant potential applied to the primary 3! is equal to the signal potential from tube 5 developed across 31 less a voltage dependent upon the potential developed between the terminal 58 and the terminal 49 or between the terminal 84 and the terminal 48. The connection to the terminal or tap 54 provides a lesser degree of feedback.

The manner in which the voltage derived from terminals 58 and 48 or 54 and 48 is related to the signal potential from tube 5 developed across 81 is the same as the relationship existing for the circuit of Figure 2 except that the input impedance of 31 is substituted for the impedance l8.

The anode circuit of the input stage 5 includes, in series, the plate impedance of the tube, the impedance of the primary winding 31 andthe input impedance of one-half of the primary 41, and anode current is supplied to both the tube 86 and the tube 5 through one-half of the primary 41, and this may, in certain cases, be balanced by suitable impedances in shunt with the other half of the transformer winding, as indicated, for example, by a resistor 56 so that the load on each half of the primary 41 is the same, as the input stage 5 places'an additional load on the primary From the foregoing description, it will be seen that inverse feedback may be provided between an output circuit and an input circuit of an amrllder stage, or of an amplifier, by utilizing'a voltage divider network comprising the output load impedance of an amplifier stage, the load impedance of the preceding stage and the tube plate impedance of said preceding stage in series, whereby inverse feedback is effected without additional coupling elements being required in the amplifier circuit.

An audio frequency amplifier comprising, in

combination, a balanced power output stage including an input transformer and an output transformer provided witha center tap, a driver stage coupled to said first-named stage, an output circuit for said driver stage including the primary of said input transformer and a connection on the primary of said output transformer in spaced relation to the center tap DONALD S. BOND. 

